6. ABSTRACT Pulmonary arterial hypertension (PAH) contributes to the morbidity and mortality of patients with lung and heart diseases, important health problems in the Veteran's population. Hypoxia induces PAH, which comprises structural remodeling of pulmonary arteries. Recently studies have found that mice deficient in Toll- like receptor 4 (TLR4) are susceptible to lethal oxidant lung injury and emphysema. Consistent with these observations that TLR4 deficiency is important in the development of lung diseases, we found that TLR4-, not TLR2-deficient mice spontaneously developed PAH. Further, hypoxia induced PAH in wild type and TLR2-/- mice, but did not enhance PAH spontaneously developed in TLR4-/- mice, suggesting that TLR4 plays an important role in maintaining normal pulmonary vasculature and hypoxia induces PAH via TLR4. This proposal is focused on understanding the role of TLR4 and its signaling pathways in regulating the development of PAH. Vascular remodeling represents a key step in the pathogenesis of PAH. At the cellular level, vascular remodeling is associated withincreased proliferation of smooth muscle cells (SMC). Preliminary studies found that hypoxia inhibited the expression of TLR4, and induced the expression of NOX1 and NOX4 in pulmonary artery SMC. On the other hand, SMC derived from TLR4-/- mice exhibited increased production of intracellular reactive oxygen species (ROS) and increased expression of NOX4 and NOX1. NOX4 is the predominant NADPH oxidase in SMC that produce ROS. Increased NOX4 promotes SMC proliferation, and inhibition of NOX4 reduces production of ROS and SMC proliferation. Therefore, we hypothesize that inhibition of TLR4 by hypoxia promotes PAH by activating NADPH oxidase signaling that increases SMC proliferation and vascular remodeling. Two specific aims will be pursued to test the hypothesis: Specific Aim 1: Characterize the functional role of TLR4 signals in regulation of PAH in mice. The role of TLR4 expression, its functional domain and its adaptor protein, MyD88, in regulating the development of spontaneous and hypoxia-induced PAH will be characterized. Specific Aim 2: Delineate the molecular mechanisms responsible for TLR4 regulation of PAH. The cross talk between TLR4 signals and the NADPH oxidase-mediated redox signaling pathways in regulating SMC proliferation and vascular remodeling will be determined. Our studies will define an important novel function of TLR4 signaling in regulating SMC proliferation and vascular remodeling in the pathogenesis of PAH, which will broaden our knowledge on the crosstalk between innate immunity and oxidative stress signals in the pulmonary vasculature. Determination of the molecular mechanisms responsible for TLR signaling cascade in the pathogenesis of PAH might lead to novel strategies to prevent or intervene PAH, and possibly other diseases induced by chronic oxidative stress and vascular remodeling. Such disorders affect many veterans, the proposed studies will help to improve quality of their life.